After purchasing a Geissele Automatics Suppressor Alignment Rod, for 5.56 mm we checked all my friends 5.56 AR's for proper suppressor alignment. Upon checking them, we found the Colt, Knights, and LWRC barrels allowed the suppressor alignment rod to freely and easily slide through the barrel from breech to bore with no friction. However, the Daniel Defense 12.5 inch barrel didn't allow the gauge rod to freely pass through the barrel. A slight nudge was needed to push the gauge rod through the barrel. It showed a partial fail for suppressor alignment. We believe the Gemtech G5 T flash hider needs adjusting, suspected misaligned shim, but the question remains, why doesn't the rod pass through freely. Aren't all mil spec barrels diameters the same ? The Daniel Defense rifle shoots fine, as do all the others. There is about 650 rounds through this barrel. The other rifles mentioned that we checked all have 14.5 and 14.7 inch barrels. Is the shorter 12.5 inch Daniel Defense barrel supposed to have tighter diameter because of a slightly shorter barrel length...I wouldn't think it would, but this is what brings us to this mighty forum....looking for some input if anyone can explain this quagmire.